This invention relates to an electronic dipole used as looping element of a telephone line, and applies more especially to a private branch exchange.
A private branch exchange allows more particularly to connect a certain number of subscribers to a central office by means of one or more telephone lines supplied from the central office. The private branch exchange calls or receives a call coming from the central office by looping the line, this operation causing the flow of a direct current. A dialed number is sent from the private branch exchange to the central office by repeated looping and breaking of the line. The looping element, placed in parallel between the two wires of the line, must therefore present on the one hand a low impedance to the d-c supply current, and on the other hand the highest possible impedance to alternating current, in order to permit the transmission of speech signals.
This looping element can be designed in the usual way with an inductance placed in the circuit by means of a relay. This device has the disadvantage of being bulky and heavy, and introduces delays and distortion during looping.
Completely electronic looping elements are also known. Such an element is described in the French patent application No. 73 40850 filed by the applicant on Nov. 16, 1973, and entitled "Electronic Trunk Junctor". This element is an electronic dipole operating at constant current, the current being fixed at a value higher than the detection threshold of the loop state detector located in the central office. Variations in the resistance of the telephone line, due mainly to its length, can be overcome by the operation at constant current for the dipole. However, the constant current operation characteristic is obtained only when a relatively high voltage (about 10 V) is present at the terminals of the dipole.
The object of this invention precisely is an electronic dipole for looping a telephone line operating at constant current even when a low voltage is applied to its terminals.
The dipole according to the invention, including a first transistor whose collector is connected directly to a first terminal of the line and whose emitter is connected to a second terminal of the line through a resistor, and a second transistor whose collector and base are connected respectively to the base and emitter of the first transistor and whose emitter is connected to the second terminal of the line, and wherein a circuit through which a constant current flows, is coupled between the collector and the base of the first transistor.